From Nadia Drake’s Ars Technica story: Mysterious Undersea “Crop Circles” Finally Explained:
In 2011, scientists determined that the rings themselves were made of eelgrass, a native type of seagrass that hosts small fish and other crustaceans. Late last year, after studying the circles, the same team found that an interaction between the eelgrasses and seafloor sulphides sculpts the mysterious shapes.
Normally, stands of eel grass grow outward in a radial pattern, forming big, circular grassy carpets. But as the grasses grow and populate shallow waters, they tend to grab onto and sequester mud that would normally continue moving along the seafloor. As the sulphide-saturated mud accumulates around the eelgrasses, the poison begins to kill the plants, starting with the older, weaker grasses in the circle’s center.
Eventually, as poisons hollow out their centers, the stands of eelgrass take on a ring-like appearance, leaving the seafloor spotted with one of the shapes that’s almost guaranteed to intrigue and perplex us